1. Field of the Invention
The present disclosure relates to magnetic resonance imaging (MRI) methods, and more particularly to MRI image reconstruction methods.
2. Description of the Related Art
Magnetic resonance imaging (“MRI”) is an important diagnostic and imaging technique for targets such as a human body. MRI techniques are based on the absorption and emission of radio frequency (RF) energy by the nuclei of atoms. Typically, the target is placed in a strong magnetic field that causes the generally disordered and randomly oriented nuclear spins of the atoms to become aligned with the applied magnetic field. One or more RF pulses are transmitted into the target, perturbing the nuclear spins. As the nuclear spins relax to their aligned state, the nuclei emit RF energy that is detected by receiving coils disposed about the target. The received RF energy is processed into a magnetic resonance (“MR”) image of a portion of the target.
By utilizing nonuniform magnetic fields having gradients in each of the three spatial dimensions, the location of the emitting nuclei can be spatially encoded so that the target can be imaged in three dimensions (“3-D”). It is common to refer to the direction of a “slice” in the target as the z-direction and the two mutually orthogonal directions in the plane of the slice as the x- and y-directions. Generally, RF pulses having a range of frequencies are transmitted into the target, and by using well-known frequency encoding (e.g., for the x-direction) and phase encoding techniques (e.g., for the y-direction), a set of MRI data is received by each of the receiver coils for each slice in the target. The MRI data are a Fourier representation (e.g., frequency domain) of the nuclear emissions from the target, and an image of the slice of the target is obtained by performing a Fourier transformation of the frequency domain MRI data. In MRI systems having multiple receiver coils (parallel MRI), an image is reconstructed from each receiver coil, and a final image is a combination of the images from each coil. Multiple receiver coil systems can be used to achieve high spatial and temporal resolution, to suppress image artifacts, and to reduce MRI scan time.
MRI data in the phase encoding direction can be acquired at the appropriate Nyquist sampling rate to avoid artifacts in the final image caused by aliasing. However, sampling at the Nyquist rate is time consuming and can prevent imaging targets that move (e.g., a beating heart). Recent methods of parallel or partially-parallel MRI (“P-MRI”) therefore undersample the phase encoding dimension (as compared to Nyquist sampling) by a reduction factor R (which may be 2, 3, 4, or more) in order to decrease data acquisition time. The undersampling results in certain data in k-space not being acquired, and therefore not available for image reconstruction. However, dissimilarities in the spatial sensitivities of the multiple receiver coils provide supplementary spatial encoding information, which is known as “sensitivity encoding.” Some of the P-MRI methods are able to reconstruct values for the unacquired data by combining undersampled, sensitivity-encoded MRI data received by different coils. By combining the acquired data and the reconstructed values of the unacquired data, a fully sampled set of k-space MRI data is produced that can be used to create a final image with reduced aliasing artifacts. Typically, parallel imaging methods reconstruct the unacquired data from the acquired data, whereas “partially” parallel imaging methods acquire additional auto-calibration signal (“ACS”) data to assist in the reconstruction.
Although, some P-MRI methods have produced good-quality images from MRI data acquired at lower reduction factors, many of the present P-MRI methods suffer from strong noise amplification and non-resolved aliasing artifacts when the MRI data is acquired at higher reduction factors. Moreover, certain P-MRI methods require time-consuming calibration scans that can provide erroneous data due to, for example, target motion between the calibration scan and the imaging scan.